The immune response is coordinated by the actions of cytokines produced from activated T lymphocytes. The precursors for most T lymphocytes arise in the bone marrow and migrate to the thymus where they differentiate and express receptors capable of interacting with antigen. These differentiated T lymphocytes then migrate to the peripheral lymphoid organs where they remain quiescent until they come in contact with the cognate antigen. The interaction of antigen with the antigen receptor on T lymphocytes initiates an ordered series of pleiotropic changes; a process denoted as T lymphocyte activation. T lymphocyte activation is a 7 to 10 day process that results in cell division and the acquisition of immunological functions such as cytotoxicity and the production of lymphokines that induce antibody production by B lymphocytes and control the growth and differentiation of granulocyte and macrophage precursors. The cytokines produced by activated T lymphocytes act upon other cells of the immune system to coordinate their behavior and bring about an effective immune response.
The initiation of T lymphocyte activation requires a complex interaction of the antigen receptor with the combination of antigen and self-histocompatibility molecules on the surface of antigen-presenting cells. T lymphocytes may also be activated by relatively simple stimuli such as the combination of a calcium ionophore (e.g., ionomycin) and an activator of protein kinase C, such as phorbol myristate acetate (PMA). Several lectins, including phytohemagglutinin (PHA) may also be used to activate T cells (Nowell (1960) Cancer Res. 20: 462).
T lymphocyte activation involves the specific regulation of particular subsets of genes. The transcriptional regulation characteristic of T cell activation begins minutes after the antigen encounter and continues until at least 10 days later. The T lymphocyte activation genes can be grouped according to the time after stimulation at which each gene is transcribed. Early genes are the first subset of T lymphocyte activation genes that is expressed during the activation process. Expression of the early genes triggers the transcriptional modulation of subsequent genes in the activation pathway. Because of the critical role of the T lymphocyte in the immune response, agents that interfere with expression of the early activation genes, such as cyclosporin A and FK506, are effective immunosuppressants.
Transcription of the early genes requires the presence of specific transcription factors, such as NF-AT, which in turn are regulated through interactions with the antigen receptor. These transcription factors are proteins which act through enhancer and promoter elements near the early activation genes to modulate the rate of transcription of these genes. Many of these transcription factors reversibly bind to specific DNA sequences located in and near enhancer elements.
The interleukin-2 (IL-2) gene is a paradigmatic early activation gene. The IL-2 gene product plays a critical role in T lymphocyte proliferation and differentiation. The IL-2 gene is transcriptionally active only in T cells that have been stimulated through the antigen receptor or its associated molecules (Cantrell and Smith (1984) Science 224: 1312). The transcriptional induction of IL-2 in activated T lymphocytes is mediated by a typical early gene transcriptional enhancer that extends from 325 basepairs upstream of the transcriptional start site for the IL-2 gene (Durand et al. (1988) Mol. Cell. Biol. 8: 1715). Other genes known to contain NF-AT recognition sites in their regulatory regions include: .gamma.-interferon, IL-4, GM-CSF, and others. This region, which is referred to herein as the IL-2 enhancer, has been used extensively to dissect the requirements for T lymphocyte activation. An array of transcription factors, including NF-AT, NFkb, AP-1, Oct-1, and a newly identified protein that associates with Oct-1 called OAP-40, bind to sequences in this region (Ullman et al. (1991) Science 254: 558). These different transcription factors act together to integrate the complex requirements for T lymphocyte activation.
Among the group of transcription factors mentioned above, the presence of NF-AT is characteristic of the transcription events involving early activation genes, in that its recognition sequence is able to enhance transcription of linked heterologous genes in activated T cells of transgenic animals (Verweij et al. (1990) J. Biol. Chem. 265: 15788). The NF-AT sequence element is also the only known transcriptional element in the IL-2 enhancer that has no stimulatory effect on transcription in the absence of physiologic activation of the T lymphocyte through the antigen receptor or through treatment of T cells with the combination of ionomycin and PMA. For example, the NF-AT element enhances transcription of linked sequences in T lymphocytes which have had proper presentation of specific antigen by MHC-matched antigen presenting cells or have been stimulated with the combination of ionomycin/PMA, but not in unstimulated T lymphocytes (Durand et al. (1988) op.cit; Shaw et al. (1988) op.cit; Karttunen and Shastri (1991) Proc. Natl. Acad. Sci. USA 88: 3972; Verweij et al. (1990) op.cit). Moreover, the NF-AT sequence element naturally enhances transcription of the IL-2 gene only in activated T lymphocytes.
Other elements within the IL-2 enhancer, for example, the NFkb site or the AP-1 site, activate transcription in response to less specific stimuli, such as tumor necrosis factor a or simply PMA by itself. These compounds do not by themselves activate transcription of the IL-2 gene and other early activation genes, and do not lead to T lymphocyte activation.
Such observations indicate that the expression of certain early genes, such as the interleukin-2 gene may be regulated by the protein complex NF-AT. Data have also indicated that a selective genetic deficiency of NF-AT produces severe combined immunodeficiency (SCID) (Chatilla et al. (1989) New Engl. J. Med. 320: 696).
One of the functional sequences in the IL-2 enhancer is a binding site for a multimeric protein complex, designated NF-AT (nuclear factor of activated T lymphocytes), that functions as a transcriptional regulator of IL-2, IL-4, and other early activation genes (Shaw et al. (1988) Science 241: 202). The NF-AT transcription complex is formed subsequent to a signal from the antigen receptor. Enhancement of transcription of genes adjacent to the NF-AT recognition site requires that the NF-AT complex bind to the recognition site (Shaw et al. (1988) op.cit). Although the molecular makeup of NF-AT is not fully defined, studies have reported that NF-AT can be reconstituted from a ubiquitous nuclear component that requires protein synthesis for induction and a T cell-specific constitutive cytoplasmic component, designated NF-AT.sub.c (Flanagan et al. (1991) Nature 352: 803). This cytoplasmic component, NF-AT.sub.c, associates with the nucleus in response to calcium signalling in a manner that is inhibited by the immunosuppressive drugs cyclosporin A (CsA) and FK506. The nuclear component of NF-AT can be induced with PMA, is not sensitive to CsA or FK506, and can be seen in cells of non-T cell origin such as HeLa and Cos.
Northrop et al. (1993) J. Biol. Chem. 268: 2917 report that the nuclear component of NF-AT contains the phorbol ester-inducible transcription factor, AP-1 (Jun/Fos), and show that antisera to Fos (a component of AP-1) inhibits NF-AT binding to DNA containing a binding site for AP-1. Moreover, Northrop et al. show that NF-AT DNA binding can be reconstituted in vitro using semi-purified AP-1 proteins mixed with cytosol from T lymphocytes, presumably containing NF-AT.sub.c. Northrop et al. also report partial purification of NF-AT.sub.c and report a molecular mass range of approximately 94 to 116 kD as estimated by SDS-polyacrylamide gel electrophoresis.
As noted above, cyclosporin A (CsA) and FK506 are capable of acting as immunosuppressants. These agents inhibit T and B cell activation, mast cell degranulation, and other processes essential to an effective immune response (Borel et al. (1976) Agents Actions 6: 468; Sung et al. (1988) J. Exp. Med. 168: 1539; Gao et al. Nature 336: 176). In T lymphocytes, these drugs disrupt a step in the signal transduction pathway(s) through which the binding of antigen to the T cell antigen receptor produces enhanced transcription of specific cytokine genes involved in the coordination of the immune response. Thus, these agents prevent T lymphocyte activation (Crabtree et al. (1989) Science 243: 355; Schreiber et al. (1989) Science 251:283; Hohman & Hutlsch (1990) New Biol. 2: 663) and act as immunosuppressants.
Putative intracellular receptors for FK506 and CsA have been described and found to be cis-trans prolyl isomerases (Fischer & Bang (1985) Biochim. Biophys. Acta 828: 39; Fischer et al. Nature 337: 476; Handschumacher et al. (1984) Science 226: 544; Lang & Schmid (1988) Nature 331: 453; Standaert et al. (1990) Nature 346: 671). Binding of the drugs inhibits isomerase activity; however, studies with other prolyl isomerase inhibitors (Bierer et al. (1990) Science 250: 556) and analysis of cyclosporin-resistant mutants in yeast suggest that the prevention of T lymphocyte activation results from formation of an inhibitory complex involving the drug and the isomerase (Bierer et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87: 9231; Tropschug et al. (1989) Nature 342: 953), and not from inhibition of the isomerase activity per se. CsA and FK-506 prevent T cell proliferation by inhibiting a calcium-dependent signalling event required for the induction of interleukin-2 transcription.
Calcineurin, a calmodulin-dependent protein phosphatase which occurs in various isoforms, has been identified as a critical component of T cell activation through the signal transduction pathway leading to transcriptional activation of NF-AT-dependent genes, such as lymphokine genes (Liu et al. (1991) Cell 66: 807; Clipstone and Crabtree (1992) Nature 357: 695; O'Keefe et al. (1992) Nature 357: 692)
Transcriptional enhancement involving NF-AT recognition sequences is completely blocked in T cells treated with efficacious concentrations of cyclosporin A or FK506, with little or no specific effect on transcriptional enhancement involving recognition sites for other transcription factors, such as AP-1 and NF-.kappa.B (Shaw et al.(1988) op.cit; Emmel et al. (1989) Science 246: 1617; Mattila et al. (1990) EMBO J. 9: 4425). This blockage can be overcome, at least partially, by the expression of hyperphysiolgical amounts of calcineurin (Clipstone and Crabtree (1992) op.cit.).
Unfortunately, while both cyclosporin A and FK506 are potent immunosuppressive agents, both drugs possess detrimental properties. For example, cyclosporin elicits adverse reactions including renal dysfunction, tremors, nausea and hypertension. Indeed, for many years researchers have attempted to develop superior replacements, with FK506 being the most recent candidate. However, without understanding the mechanisms by which cyclosporin (or FK506) functions at the intracellular level, developing improved immunosuppressants represents an extremely difficult research effort with a limited likelihood of success.
Thus, there exists a significant need to understand the functional basis of T cell activation involving NF-AT, particularly with regard to the mechanism by which these immunosuppresants such as CsA and FK506 inhibit transcription of the early activation genes. With such knowledge, improved assays for screening drug candidates would be feasible, which could in turn dramatically enhance the search process. Modulation of the immune system, especially modulation of T cell activation, also may be effected by directly altering the amount or activity of NF-AT. The present invention fulfills these and other needs.
The references discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention.